How many hours of recorded speech?

[Lindsay Marean]

I'm helping to document a language with few first-language speakers 
living.  We want to record them speaking naturally (and transcribe and 
translate the recordings), and we hope to use this documentation as the 
basis for more language description in the future.

I'm looking for people's opinions, experiences, and citations - how many 
hours of recorded speech are minimally "enough" to most likely represent 
the grammar of the language?  Are there particular discourse types that 
we should be certain to record, besides narratives and conversations?

Best regards,
Lindsay